The present invention relates to a tension member and a method for installing the tension member as a tendon or tether for a tension leg platform. The tension member of the invention is intended primarily to be used in connection with tendons for tension leg platforms, but other applications are also possible, such as stays or wires for bridges, (e.g., suspension bridges or cable-stayed bridges), the bracing of tunnels or other applications where there is a need for a lightweight, strong wire or stay. The invention is therefore not limited to the application described below.
Tension leg platforms are widely used in drilling and production on oil fields where for various reasons it is not possible or financially viable to install a fixed platform, and where it would not be expedient to use a floating platform moored by means of anchors and anchor chains.
Tension leg platforms are in principle floating platforms, where, however, instead of a slack mooring by means of anchors and anchor chains, tendons extend from the platform approximately vertically down to an anchorage on the seafloor. The tendons are put under considerable tension to ensure that the platform stays as much as possible in the same position relative to the seafloor. The stable position of the platform is of great advantage for both drilling and production. However, this makes heavy demands on the tendons used, their attachment to the platform and the anchorage on the seafloor.
Today""s tendon tension legs consist of steel pipes in sections. The sections may be of different lengths and different diameters and have different wall thicknesses. Insofar as strength is concerned, it is an advantage for the steel pipes to have a large wall thickness, but as regards weight and thus also the load on the attachment to the platform, it is an advantage if the wall thickness is small. Wall thickness will therefore always be chosen as a compromise between strength and weight. These steel tendons function well to moderate depths, i.e., depths of a few hundred meters. However, oil and gas production is now taking place at ever-greater depths, often up to 2000 m. Under such conditions heavy demands are made on the strength of the tendons, and tendons of steel are not usable. On account of the increased strength requirement, the wall thickness would have to be very large and the pipes would thus be extremely heavy. To facilitate transport, they would also have to consist of very many sections which would need to be joined together during installation. The tendons would thus have a considerable number of joints, which would also add to the substantial increase in weight. To counteract this increase in weight it would therefore be necessary to equip the tendons with a large number of floats. All this would result in a very costly and heavy installation.
Carbon fibers, with their low weight and high tensile strength, have already been used in various areas in connection with oil and gas recovery, for example, as hoisting cable for great depths, where the weight of a hoisting cable in steel would create problems.
According to the present invention, one of the objects is to exploit the advantageous properties of the carbon fibers, in particular their great strength when subjected to tensile stress, also when used in tendons. However, carbon fibers also have one considerable negative property; they have very small breaking strength when subjected to shearing stress. When constructing a tendon consisting of carbon fibers, this will have to be taken into account.
During the development of the present invention ideas were taken from the Applicant""s own pipe bundle cable as described in NO 155826. In this publication, several smaller piplines are placed in a bundle in a way that makes it possible for them to move axially with respect to each other. The cable is, however, not able to endure large tensions.
NO 174940 describes a method and a machine for making a cable string of several tubings or cables. This cable string includes a center pipe. This cable string is will not endure large tensions.
EP 685 592 describes a method for separating individual strands in a steel wire to prevent wear and increase the cross section. The plastic elements between the strands will be squeezed when the cable is loaded, and thus contact between the strands is prevented. The strands are not freely axially moveable relative to each other because of this squeezing.
FR 2078622 also describes a steel wire where a filler is put in to separate the individual wires. Free axial movement of the strands is difficult because direct contact between the strands occurs.
U.S. Pat. No. 3,088,269 describes a method for producing a steel wire with a smooth surface for use in ropeways etc. Filler elements are laid in between the strands to fix these and keep them separated from each other. Free movement between the strands is not possible, since the aim is to obtain a squeezing between the strands and the filler elements.
According to the present invention, one of the objects is to provide a tendon of preferably carbon fibers, which can be used for tension leg platforms at great depths, where the carbon fibers are protected against shearing stress. However, other fiber materials having approximately the same properties as carbon fibers may also be used, for example, glass fibers.
The invention will now be described in more detail with reference to the accompanying drawings, wherein:
FIG. 1 is a perspective view of a tension leg platform;
FIG. 2 is a sectional view through a tension member according to a first embodiment of the invention;
FIG. 3 is a sectional view through a tension member according to a second embodiment of the invention; and
FIG. 4 is a sectional view through a tension member according to a third embodiment of the invention.